The present invention relates to a process for the synthesis of bis-(substituted-4-quinolyl) disulphides.
Substituted bis-quinolyl disulphides are known for antimalarial activity and some of them are used as analytical reagents. In recent studies it is stated that these compounds have selective and efficacious antibacterial activity on bacteria belonging to the Helicobacter and are also useful for the treatment or prevention of recurrence of peptic ulcer and chronic stomach inflammation accompanied by H bacteria infection.
Reference is made to Japan Patent, JP Appl. 95/14,233, 31 Jan. 1995 PCT Int.Appl. WO96 23,774 (Cl. C07D215/36),8 Aug. 1996, CA:125,247631h; (japan) wherein quinoline 4(1H)-thione was oxidized to disulphide using potassium ferricyanide. This process has draback of preparing thione and using expensive potassium ferricyanide.
Reference is made to Latv. PSR Zinat. Acad. Vestis, Kim. Ser. 1979,(9),181(Russ), CA:91,39289c and Latv. PSR Zinat. Acad. Vestis, Kim. Ser. 1979,(3),282(Russ),CA:91,193129j wherein chlorosulphonyl derivative of quinoline is converted to thiol by reduction which on oxidation gave disulphide. This process has draback of using hazardous and corrosive chlorosulphonic acid.
Reference is further made to Latv. PSR Zinat. Acad. Vestis, Kim. Ser. 1980,(4)489 (Russ),CA: 94,15526r which involve the diazotization of amino quinoline followed by reaction with thiourea gives thiol. The oxidation of thiol gives disulphide. In this process, crucial diazotization reaction requires great precautions.
Reference is also made to Pharmazine 1978,33(9),572, CA: 89, 215199v which involves the reaction of chloroquinoline with thioacetamide or thiourea to form thiol derivatives which on further oxidation gives disulphide. This particular process has drawback of using higher temperature.
All the earlier processes involve the use of corrosive and hazardous chlorosulphonic acid, crucial diazotization, and higher reaction temperature. Moreover, the isolation of thiol from aqueous medium is tedious. Although practiceable in laboratory, all these methods have little feasibility on commercial scale in term of steps involved, cost, handling of hazardous reagents and waste effluents. Therefore existing methods for the preparation of bis-quinolyl disulphides have following disadvantages.
1. Handling of hazardous and corrosive chlorosulphonic acid is difficult.
2. Diazotization reaction require great precaution.
3. Isolation of thiol from aqueous medium is troublesome.
4. Involve additional oxidation step
5. Require higher reaction temperature.
6. Number of steps involved are more.
The main object of the present invention is to provide a process for the preparation of bis(substituted-4-quinolyl) disulphide using substituted-4-haloquinolines and the sodium, potassium or ammonium salts of dithiocarbamic acid which avoids the drawbacks as detailed above.
Another object of the present invention is to develop a process for bis(substituted-4-quinolyl) disulphides without the use of hazardous chlorosulphonyl derivatives and additional oxidizing reagent.
Yet another object of the invention is to avoid the crucial diazotization and isolation of thiol from aqueous medium.
Yet another object of the invention is to use inexpensive, non-hazardous salt of dithiocarbamic acid which reduces the number of steps, minimize the waste effluent in large scale.
Yet another object of the invention is to provide a process for the preparation of bis(substituted-4-quinolyl) disulphide wherein the byproduct disubstituted thiourea is also useful as value added product, thereby improving the economy of the process.
It is yet another object of the invention to provide a process for the preparation of bis(substituted-4-quinolyl) disulphide which avoids the use of hazardous and corrosive chlorosulphonic acid, excessive and stringent precautions required for diazotization, difficulty in isolation of thiol from aqueous medium, avoids the additional oxidation step and avoids the requirement of higher reaction temperature.
The present process is useful for the one pot preparation of bis(substituted-4-quinolyl) disulphides using substituted-4-haloquinoline and sodium, potassium or ammonium salt of dithiocarbamic acid in aprotic solvent. Further aim of the process is to avoid the use of corrosive chlorosulphonyl chloride or expensive oxidizing reagents or isolation of unstable thiols. The important feature of the invention is that disulphides are prepared in one step which avoids the number of troublesome steps and use of hazardous corrosive chemicals. The yield of the disulphides is also high
Accordingly, the present invention provides a one step process for the preparation of bis-(substituted-4-quinolyl) disulphide of the formula 1 
wherein R1 is selected from the group consisting of trifluoro, trichloro, cyano and substituted amide; R2 is selected from the group consisting of alkyl, alkoxy, halogen, alkylamino; comprising reacting substituted-4-haloquinoline and a salt of alkyl or aryl dithiocarbamic acid in an organic solvent to obtain said bis-(substituted-4-quinolyl) disulphide of the formula 1 and disubstituted thiourea of the formula 2 
In one embodiment of the invention the halo in the substituted-4-haloquinoline is selected from the group consisting of iodo, bromo, chloro and fluoro.
In another embodiment of the invention, the substituents on quinoline at 2-position are selected from the group consisting of trifluoromethyl, trichloromethyl, cyano and substituted amide.
In yet another embodiment of the invention, the substituents on quinoline at the 5,6,7 or 8 position are selected from the group consisting of halogen, alkyl, alkoxy and alkylamino.
In a further embodiment of the invention, the salt of alkyl or aryl dithiocarbamic acid is selected from the group consisting of sodium, potassium and ammonium salts.
In another embodiment of the invention, the organic solvent is selected from group consisting of dimethylformamide, dimethylsulphoxide, mixture thereof and acetonitrile.
In a further embodiment of the invention, the reaction is carried out at a temperature in the range of 20xc2x0 C. to 40xc2x0 C.
In a further embodiment of the invention, the reaction is carried out for a time period in the range of 6 to 17 hrs.
In yet another embodiment of the invention, the yield of the bis-(substituted-4-quinolyl) disulphide of the formula 1 is in the range of 70 to 80%.
The present process is useful for the one pot preparation of bis(substituted-4-quinolyl) disulphides using substituted-4-haloquinoline and sodium, potassium or ammonium salt of dithiocarbamic acid in aprotic solvent. Further aim of the process is to avoid the use of corrosive chlorosulphonyl chloride or expensive oxidizing reagents or isolation of unstable thiols. The important feature of the invention is that disulphides are prepared in one step which avoids the number of troublesome steps and use of hazardous, corrosive chemicals. The yield of the disulphides is also high
Scheme 1 represent the preparation of bis(substituted-4-quinolyl) disulphide 
To overcome with the difficulties associated with earlier processes for the preparation of bis(substituted-4-quinolyl) disulphide, the inventors of present invention have disclosed the process for the preparation of bis-(substituted-4-quinolyl) disulphides using substituted 4-haloquinolines and the sodium, potassium and ammonim salt of alkyl or aryl dithiocarbamic acid in polar solvent. The disulphide formation occurs in one step, which was isolated by filtration or extraction.